Purification of carbon dioxide from a mixture of gases

ABSTRACT

A method for purification of carbon dioxide from a mixture of gases is disclosed. The method generally includes steps (A) and (B). Step (A) may bubble the gases into a solution of an electrolyte and a catalyst in an electrochemical cell. The electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode generally reduces the carbon dioxide into one or more compounds. The anode may oxidize at least one of the compounds into the carbon dioxide. Step (B) may separate the carbon dioxide from the solution.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/315,665, filed Mar. 19, 2010, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to gas purification generally and, moreparticularly, to a method and/or apparatus for implementing purificationof carbon dioxide from a mixture of gases.

BACKGROUND OF THE INVENTION

The combustion of fossil fuels in activities such as the electricitygeneration, transportation, and manufacturing produces billions of tonsof carbon dioxide annually. Research since the 1970s indicatesincreasing concentrations of carbon dioxide in the atmosphere may beresponsible for altering the Earth's climate, changing the pH of theocean and other potentially damaging effects. Countries around theworld, including the United States, are seeking ways to mitigateemissions of carbon dioxide.

In order to capture carbon dioxide from industrial sources, such as acoal-fired power plant, the carbon dioxide is separated from flue gases,which are primarily nitrogen and water and include other trace gases,metals and particulates. Previous work in the field has manylimitations, in particular the energy consumed in separating the carbondioxide from the other gases and the amount of water used in theseparation. A common technique currently available uses monoethyl amine(MEA) adsorption of the carbon dioxide from the flue gases. However, thetechnique utilizes high temperature steam to effectively separate thecarbon dioxide from the amine. As such, the technique can consume asmuch as 30% of the energy generated at a coal-fired power plant.Furthermore, carbon dioxide capture increases both the amount of waterthat is brought into a power plant and the amount of water evaporatedinto the atmosphere by the power plant. Adding the carbon dioxidecapture can increase the water brought into the power plant by 2300 to4500 liters per megawatt-hour. Increased water evaporation from thepower plant due to the carbon dioxide capture can range from 1900 to3400 liters per megawatt-hour.

Work has also been done on electrochemical systems, such aselectrodialysis via carbonates, to separate the carbon dioxide from theother gases. The electrochemical systems have slow kinetics and lowefficiency making the systems uneconomical. Membrane separation of thecarbon dioxide is possible, but no effective membranes have been made todate. Carbon dioxide is also removed by cooling the flue gas until dryice is formed. However, the energy used in the process is higher thanfor amine adsorption.

Existing processes incorporating ethyl amines or other absorbents uselarge quantities of energy and water that make such techniquesuneconomical. Membrane systems lack the strength and/or stability tolast for long periods of time. Membrane systems and electrodialysissystems also have slow rates of reaction making upscaling difficulteconomically.

SUMMARY OF THE INVENTION

The present invention concerns a method for purification of carbondioxide from a mixture of gases. The method generally includes steps (A)and (B). Step (A) may bubble the gases into a solution of an electrolyteand a catalyst in an electrochemical cell. The electrochemical cell mayinclude an anode in a first cell compartment and a cathode in a secondcell compartment. The cathode generally reduces the carbon dioxide intoone or more compounds. The anode may oxidize at least one of thecompounds into the carbon dioxide. Step (B) may separate the carbondioxide from the solution.

The objects, features and advantages of the present invention includeproviding a method and/or apparatus for implementing purification ofcarbon dioxide from a mixture of gases that may (i) utilize loweramounts of energy than conventional techniques, (ii) provide reactionrates sufficiently high for scalability, (iii) provide stabile long-termreduction of carbon dioxide using copper-based alloys electrodes, (iv)provide for commercialization of electrochemical purification of carbondioxide from a mixed gas and/or (v) consume little to no water in thepurification chemistry.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

FIG. 1 is a block diagram of a system in accordance with a preferredembodiment of the present invention;

FIG. 2 is a flow diagram of an example method for separating carbondioxide from a mixture of gases;

FIG. 3 is a formula of an aromatic heterocyclic amine catalyst;

FIGS. 4-6 are formulae of substituted or unsubstituted aromatic 5-memberheterocyclic amines or 6-member heterocyclic amines; and

FIG. 7 is a table illustrating relative product yields for differentcathode material, catalyst, electrolyte, pH level and cathode potentialcombinations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before any embodiments of the invention are explained in detail, it isto be understood that the embodiments may not be limited in applicationper the details of the structure or the function as set forth in thefollowing descriptions or illustrated in the figures of the drawing.Different embodiments may be capable of being practiced or carried outin various ways. Also, it is to be understood that the phraseology andterminology used herein is for the purpose of description and should notbe regarded as limiting. The use of terms such as “including,”“comprising,” or “having” and variations thereof herein are generallymeant to encompass the item listed thereafter and equivalents thereof aswell as additional items. Further, unless otherwise noted, technicalterms may be used according to conventional usage.

In the following description of methods, process steps may be carriedout over a range of temperatures (e.g., approximately 10° C. (Celsius)to 50° C.) and a range of pressures (e.g., approximately 1 to 10atmospheres) unless otherwise specified. Numerical ranges recited hereingenerally include all values from the lower value to the upper value(e.g., all possible combinations of numerical values between the lowestvalue and the highest value enumerated are considered expressly stated).For example, if a concentration range or beneficial effect range isstated as 1% to 50%, it is intended that values such as 2% to 40%, 10%to 30%, or 1% to 3%, etc., are expressly enumerated. The above may besimple examples of what is specifically intended.

Some embodiments of the present invention generally separate carbondioxide from a mixture of gases using a reduction and oxidation process.The carbon dioxide may be isolated from the mixed gas by reducing thecarbon dioxide to one or more compounds at a cathode. The compounds mayinclude, but are not limited to oxalate, oxalate salts and/or organicacids. The organic acids may include, but are not limited to, oxalicacid, formic acid and glyoxylic acid. The compounds may be oxidized toform carbon dioxide at an anode. The resulting pure, or nearly pure,carbon dioxide may be subsequently collected for storage and/or otheruses.

The formation of oxalate may be maximized in some embodiments. Evolutionof oxalate from carbon dioxide may be achieved with a single electronper carbon atom. The evolution of other organic molecules generallyinvolves two or more electrons per carbon atom. Therefore, the amount ofelectrical energy used to make oxalate may be smaller than other organicmolecules.

The purification of the carbon dioxide may be achieved efficiently in adivided electrochemical cell in which (i) a compartment contains ananode and (ii) another compartment contains a working cathode electrodeand a catalyst. The compartments may be separated by an optional porousglass frit or other ion conducting bridge. Both compartments generallycontain an aqueous solution of an electrolyte. A mixed gas containingthe carbon dioxide may be continuously bubbled through the cathodicelectrolyte solution to saturate the solution.

The mixed gas may be obtained from any sources (e.g., an exhaust streamfrom fossil-fuel burning power or industrial plants, from geothermal ornatural gas wells or the atmosphere itself). Generally, the mixed gasesmay be obtained from concentrated point sources of generation prior tobeing released into the atmosphere. For example, high concentrationcarbon dioxide generally exists in flue gases of fossil fuel (e.g.,coal, natural gas, oil, etc.) burning power plants. Emissions fromvaried industries, including geothermal wells, may also be capturedon-site.

Referring to FIG. 1, a block diagram of a system 100 is shown inaccordance with a preferred embodiment of the present invention. Thesystem (or apparatus) 100 generally comprises a cell (or container) 102,a liquid source 104, a power source 106, a gas source 108 and anoptional valve 111. An output gas may be presented from the cell 102 atthe valve 111.

The cell 102 may be implemented as a divided cell or an undivided cell.The divided cell may be a divided electrochemical cell and/or a dividedphotochemical cell. The cell 102 is generally operational to separatecarbon dioxide (CO₂) from a mixture of gases. The purification generallytakes place by bubbling the mixed gases into an aqueous solution of anelectrolyte in the cell 102. A cathode in the cell 102 may reduce thecarbon dioxide and protons into the one or more compounds. An anode inthe cell 102 generally oxidizes the compounds back into the carbondioxide.

The cell 102 generally comprises one or more compartments (or chambers)114 a-114 b, an optional separator (or membrane) 116, an anode 118 and acathode 120. The anode 118 may be disposed in a given compartment (e.g.,114 a). The cathode 120 may be disposed in another compartment (e.g.,114 b) on an opposite side of the separator 116 as the anode 118. Anaqueous solution 122 may fill all of the compartments 114 a-114 b. Acatalyst 124 may be added to the compartment 114 b containing thecathode 120.

The liquid source 104 may implement a water source. The liquid source104 may be operational to provide pure water to the cell 102.

The power source 106 may implement a variable voltage source. The source106 may be operational to generate an electrical potential between theanode 118 and the cathode 120. The electrical potential may be a DCvoltage. In some embodiments, a range of the electrical potential may bebetween −0.7 volts and −1 volt.

The gas source 108 may implement a mixed gas source. The source 108 isgenerally operational to provide a mixture of gasses, including carbondioxide to the cell 102. The source 108 may also be operational topressurize the cell 102. In some embodiments, the gas is bubbleddirectly into the compartment 114 b containing the cathode 120.

The valve 111 may be implemented as a pressure relief valve. In apressurized type of cell 102, the valve 111 may be used at a pointoutside the cell 102. While the valve 111 is closed, the carbon dioxidemay be trapped in the cell 102. While the valve 111 is open, pressurizedcarbon dioxide gas may leave the cell 102 through a port 128. Pressurein the cell 102 generally allows the pressurized carbon dioxide gas toseparate from the electrolyte 122. In an unpressurized type of cell 102,bubbles of carbon dioxide generally form at the anode 118 whenconcentrations of the carbon dioxide exceed a threshold (e.g.,approximately 33 millimolar (mM) at 25° C.). The resulting carbondioxide gas may leave the cell 102 through the port 128.

In the process described, the carbon dioxide is reduced to one or morecompounds (e.g., oxalate, oxalate salts and/or organic acids) at thecathode 120 while the compounds may be oxidized back into carbon dioxideat the anode 118. The electrolyte 122 in the cell 102 may use water as asolvent with any salts that are water soluble and with a heterocyclecatalyst 124. The electrolyte 122 in the cell 102 may use water as asolvent with any salts that are water soluble and with a pyridine orpyridine-derived catalyst 124. The catalysts 124 may include, but arenot limited to, nitrogen, sulfur and oxygen containing heterocycles.Examples of the heterocyclic compounds may be pyridine, imidazole,pyrrole, thiazole, furan, thiophene and the substituted heterocyclessuch as amino-thiazole and benzimidazole. Cathode materials generallyinclude any conductor. Any anode material with a low overpotential foroxidation may be used. The low overpotential may also be used tominimize or eliminate the evolution of oxygen from water at the anode118. The overall process is generally driven by the power source 106.Combinations of anodes 118, cathodes 120, electrolytes 122, catalysts124, introduction of mixed gas into the cell 102, pH levels and theelectric potential from the power source 106 may be used to control thereaction in the cell 102.

In some nonaqueous embodiments, the solvent may include methanol,acetonitrile, and/or other nonaqueous solvents. The electrolytesgenerally include tetraalkyl ammonium salts and a heterocyclic catalyst.A primary product may be oxalate in a completely nonaqueous system. In asystem containing nonaqueous catholyte and aqueous anolyte, thecompounds generally include all of the compounds seen in aqueous systemswith higher yields.

The electrodes may be a suitable conductive electrode, such as Al, Au,Ag, C, Cd, Co, Cr, Cu, Cu alloys (e.g., brass and bronze), Ga, Hg, In,Mo, Nb, Ni, Ni alloys, Ni—Fe alloys, Sn, Sn alloys, Ti, V, W, Zn,stainless steel (SS), austenitic steel, ferritic steel, duplex steel,martensitic steel, Nichrome, elgiloy (e.g., Co—Ni—Cr), degeneratelydoped p-Si, degenerately doped p-SI:As and degenerately doped p-Si:B.Other conductive electrodes may be implemented to meet the criteria of aparticular application.

The catalyst may be one or more substituted aromatic heterocyclic aminesor unsubstituted aromatic heterocyclic amines as homogeneous catalystsin the aqueous solution. A concentration of the catalyst may be about 1mM to 1 M. Suitable amines are generally heterocycles which may include,but are not limited to, heterocyclic compounds that are 5-member or6-member rings with at least one ring nitrogen. Aromatic heterocyclicamines may include, but are not limited to, unsubstituted andsubstituted pyridines and imidazoles. Substituted pyridines andimidazoles may include, but are not limited to mono and disubstitutedpyridines and imidazoles. For example, suitable catalysts may includestraight chain or branched chain lower alkyl (e.g., C1-C10) mono anddisubstituted compounds such as 2-methylpyridine, 4-tertbutyl pyridine,2,6-dimethylpyridine (2,6-lutidine); bipyridines, such as4,4′-bipyridine; amino-substituted pyridines, such as 4-dimethylaminopyridine; and hydroxyl-substituted pyridines (e.g., 4-hydroxy-pyridine)and substituted or unsubstituted quinoline or isoquinolines. Catalystsmay also suitably include substituted or unsubstituted dinitrogenheterocyclic amines, such as pyrazine, pyridazine and pyrimidine. Othercatalysts generally include azoles, imidazoles, indoles, oxazoles,thiazoles, substituted species and complex multi-ring amines such asadenine, pterin, pteridine, benzimidazole, phenonthroline and the like.

The electrolyte may be suitably a salt, such as KCl, NaNO₃, Na₂SO₄,NaCl, NaF, NaClO₄, KClO₄, K₂SiO₃, or CaCl₂ at a concentration of about0.5 M. A concentration of the electrolytes may range from about 0.1 M to1M. Other electrolytes may include, but are not limited to, all group 1cations (e.g., H, Li, Na, K, Rb and Cs) except Francium (Fr), Ca,ammonium cations, alkylammonium cations and alkyl amines. Additionalelectrolytes may include, but are not limited to, all group 17 anions(e.g., F, Cl, Br, I and At), borates, carbonates, nitrates, nitrites,perchlorates, phosphates, polyphosphates, silicates and sulfates. Nagenerally performs as well as K with regard to best practices, so NaClmay be exchanged with KCl. NaF may perform about as well as NaCl, so NaFmay be exchanged for NaCl or KCl in many cases. Larger anions tend tochange the chemistry and favor different products. For instance, sulfatemay favor polymer or methanol production while Cl may favor productssuch as acetone. The pH of the solution is generally maintained at aboutpH 3 to 8, suitably about 4.7 to 5.6.

The process is generally controlled to get a desired product (e.g.,oxalate) by using combinations of specific conductive cathodes,catalysts, electrolytes, surface morphology of the electrodes and/orintroduction of reactants relative to the cathode. Faradaic yields forthe products generally range from less than 1% to more than 90% (e.g.,up to 100%).

Referring to FIG. 2, a flow diagram of an example method 140 forseparating carbon dioxide from a mixture of gases is shown. The method140 generally comprises a step (or block) 142, a step (or block) 144, astep (or block) 146, a step (or block) 148, a step (or block) 150 and astep (or block) 152. The method (or process) 140 may be implemented bythe system 100.

In the step 142, a mixture of gases that includes carbon dioxide may becaptured. The mixed gas may be bubbled into the chamber 114 b in thestep 144. The carbon dioxide may react with the cathode 120 in the step146 and be converted into one or more compounds (e.g., oxalate, oxalicacids and/or oxalate salts). Where the compound is oxalate, a reactionat the cathode 120 may be represented as follows:

2CO₂+2 e−→C₂O₄ ²⁻

The compounds may be transported from the compartment 114 b to thecompartment 114 a in the step 148. In an undivided type of cell 102,movement of the compounds generally takes place through iontransportation. The negatively charged compounds (e.g., oxalate²⁻) maybe transported from the cathode 120 to the anode 118 by the positivecharge at the anode 118. In a divided type of cell 102, an anionselective membrane 116 may also be employed to selectively transport thecompounds from the compartment 114 b to the compartment 114 a.Transportation may also be aided by mechanical agitation (e.g.,stirring) of the electrolyte 122 and/or other common methods. Once inthe compartment 114 a, at least one of the compounds may be oxidizedback into carbon dioxide by the anode 118 in the step 150. An oxidationof oxalate at the anode 118 may be represented as follows:

C₂O₄ ²⁻→2CO₂+2e

In the step 152, the purified (recovered) carbon dioxide may beextracted (separated) from the electrolyte 122.

The calculated energy consumed in a scaled system 100 is generally lessthan 500 kilowatt hours (kWh) per ton of carbon dioxide. In someembodiments, the energy consumed may be as low as 200 kWh per ton ofcarbon dioxide. The calculated energy generally compares favorably to a600 kWh per ton rate using existing solutions.

Some process embodiments of the present invention for making/convertinghydrocarbons generally consume a small amount of water (e.g.,approximately 1 to 3 moles of water) per mole of carbon fixed.Therefore, the processes may be a few thousand times more waterefficient than existing biofuel production techniques. With thepurification process described above, little to no water is generallyconsumed after the initial solution has been established in the cell102. The oxalic acids and/or oxalate salts may be created at the cathode120 from the carbon dioxide in the mixed gas and converted back intocarbon dioxide at the anode 118 by direct oxidation. No water may beconsumed from the chemistry, though small losses may occur in plantoperations.

Cell design and cathode treatment (e.g., surface morphology or surfacetexture) may both affect product yields and current density at thecathode 120. For instance, a divided cell 102 with a stainless steel2205 cathode 120 in a KCl electrolyte 122 generally has higher yieldswith a heavily scratched (rough) cathode 120 than an unscratched(smooth) cathode 120. Matte tin generally performs different than brighttin. Maintaining the mixed gas bubbling only on the cathode side of thedivided cell 102 (e.g., in compartment 114 b) may also alter yields.

The cell potential may alter product yields in some cases. By way ofexample, using a combination of a stainless steel 2205 cathode 120 withan imidazole catalyst 124 and a 0.5 M KCl electrolyte 122, yieldsgenerally shift from primarily formic acid to primarily acetone andethanol by lowering the cathode potential from −1.06 volts to −0.96volts.

Faradaic yields of the compounds may be improved by controlling theelectrical potential of the reaction. By maintaining a constantpotential at the cathode 120, hydrogen evolution is generally reducedand faradaic yields of the compounds increased. Addition of hydrogeninhibitors, such as acetonitrile, certain heterocycles, alcohols, andother chemicals may also increase yields of compounds.

With some embodiments, stability may be improved with cathode materialsknown to poison rapidly when reducing carbon dioxide. Copper andcopper-alloy electrodes commonly poison in less than an hour ofelectrochemically reducing carbon dioxide. However, when used with aheterocyclic amine catalyst, copper-based alloys operated for many hourswithout any observed degradation in effectiveness. The effects may beparticularly enhanced by using sulfur containing heterocycles.

Heterocycles other than pyridine may catalytically reduce carbon dioxidein the electrochemical process using many aforementioned cathodematerials, including tin, steels, nickel alloys and copper alloys.Nitrogen-containing heterocyclic amines shown to be effective include4,4′-bipyridines, picolines (methyl pyridines), 1utidines (dimethylpyridines), hydroxy pyridines, imidazole, benzimidazole, methylimidazole, pyrazine, pyrimidine, pyridazine, pyridazineimidazole,nicotinic acid, quinoline, adenine, azoles, indoles and 1,10phenanthroline. Sulfur containing heterocycles include thiazole,aminothiazoles, thiophene. Oxygen containing heterocycles include furanand oxazole. As with pyridine, the combination of catalyst, cathodematerial and electrolyte may be used to control the reactions.

Referring to FIG. 3, a formula of an aromatic heterocyclic aminecatalyst is shown. The ring structure may be an aromatic 5-memberheterocyclic ring or 6-member heterocyclic ring with at least one ringnitrogen and is optionally substituted at one or more ring positionsother than nitrogen with R. L may be C or N. R1 may be H. R2 may be H ifL is N or R2 is R if L is C. R is an optional substitutent on any ringcarbon and may be independently selected from H, a straight chain orbranched chain lower alkyl, hydroxyl, amino, pyridyl, or two R′ s takentogether with the ring carbons bonded thereto are a fused six-memberaryl ring and n=0 to 4.

Referring to FIGS. 4-6, formulae of substituted or unsubstitutedaromatic 5-member heterocyclic amines or 6-member heterocyclic aminesare shown. Referring to FIG. 4, R3 may be H. R4, R5, R7 and R8 aregenerally independently H, straight chain or branched chain lower alkyl,hydroxyl, amino, or taken together are a fused six-member aryl ring. R6may be H, straight chain or branched chain lower alkyl, hydroxyl, aminoor pyridyl.

Referring to FIG. 5, one of L1, L2 and L3 may be N, while the other L′ smay be C. R9 may be H. If L1 is N, R10 may be H. If L2 is N, R11 may beH. If L3 is N, R12 may be H. If L1, L2 or L3 is C, then R10, R11, R12,R13 and R14 may be independently selected from straight chain orbranched chain lower alkyl, hydroxyl, amino, or pyridyl.

Referring to FIGS. 6, R15 and R16 may be H. R17, R18 and R19 aregenerally independently selected from straight chain or branched chainlower alkyl, hydroxyl, amino, or pyridyl.

Some embodiments of the present invention may be further explained bythe following examples, which should not be construed by way of limitingthe scope of the invention.

Example 1 General Electrochemical Methods

Chemicals and materials. All chemicals used were >98% purity and used asreceived from the vendor (e.g., Aldrich), without further purification.Either deionized or high purity water (Nanopure, Barnstead) was used toprepare the aqueous electrolyte solutions.

Electrochemical system. The electrochemical system was composed of astandard two-compartment electrolysis cell 102 to separate the anode 118and cathode 120 reactions. A 0.5M CaCl₂/KCl was generally used as thesupporting electrolyte 122. The cathode 120 was ferritic steel and theanode 118 was a mixed metal oxide. A concentration of 30 mM imidazolewas used as the catalyst 124. Carbon dioxide was bubbled into thecathode compartment 114 b. An evolution of oxalate was generallyobserved in the cell 102.

The working electrode was of a known area. Before and during allelectrolysis, the reactants were continuously introduced into theelectrolyte to saturate the solution. The resulting pH of the solutionwas maintained at about pH 3 to pH 8, suitably, pH 4.7 to pH 5.6,depending on the aromatic heterocyclic amine employed. For example, thepH levels of 10 mM solutions of 4-hydroxy pyridine, pyridine and4-tertbutyl pyridine were 4.7, 5.28 and 5.55, respectively.

Referring to FIG. 7, a table illustrating relative product yields fordifferent cathode material, catalyst, electrolyte, pH level and cathodepotential combinations is shown. The combinations listed in the tablegenerally are not the only combinations providing a given product. Thecombinations illustrated may demonstrate high yields of the products atthe lowest potential (e.g., <1 volt amplitude).

Example 2 Analysis of Products of Electrolysis

Electrochemical experiments were generally performed using a CHInstruments potentiostat or a DC power supply with current logger to runbulk electrolysis experiments. The CH Instruments potentiostat wasgenerally used for cyclic voltammetry. Electrolysis was run underpotentiostatic conditions from approximately 6 hours to 30 hours until arelatively similar amount of charge was passed for each run.

Gas Chromatography. The electrolysis samples were analyzed using a gaschromatograph (HP 5890 GC) equipped with a FID detector. Removal of thesupporting electrolyte salt was first achieved with an Amberlite IRN-150ion exchange resin (cleaned prior to use to ensure no organic artifactsby stirring in a 0.1% v/v aqueous solution of Triton X-100, reduced(Aldrich), filtered and rinsed with a copious amount of water, andvacuum dried below the maximum temperature of the resin (approximately60° C.) before the sample was directly injected into the GC which houseda DB-Wax column (Agilent Technologies, 60 m, 1 micrometer (μm) filmthickness). Approximately 1 gram of resin was used to remove the saltfrom 1 milliliter (mL) of the sample. The injector temperature was heldat 200° C., the oven temperature maintained at 120° C., and the detectortemperature at 200° C.

Mass spectrometry. Mass spectral data was also collected to identify allorganic compounds. In a typical experiment, the sample was directlyleaked into an ultrahigh vacuum chamber and analyzed by an attached SRSResidual Gas Analyzer (with the ionizer operating at 70 electron-voltsand an emission current of 1 mA). Samples were analyzed against standardmethanol spectra obtained at the same settings to ensure comparablefragmentation patterns. Mass spectral data confirmed the presence ofmethanol and proved that the initial solution before electrolysiscontained no reduced CO₂ species. Control experiments also showed thatafter over 24 hours under illumination the epoxy used to insulate thebackside of the electrode did not leach any organic material that wouldgive false results for the reduction of CO₂. NMR spectra of electrolytevolumes after illumination were obtained using an automated BrukerUltrashield™ 500 Plus spectrometer with an excitation sculpting pulsetechnique for water suppression. Data processing was achieved usingMestReNova software. For methanol standards and electrolyte samples, therepresentative signal for methanol was observed between 3.18 to 3.30parts per million (ppm).

Nuclear Magnetic Resonance. NMR spectra of electrolyte volumes afterbulk electrolysis were also obtained using an automated BrukerUltrashield™ 500 Plus spectrometer with an excitation sculpting pulsetechnique for water suppression. Data processing was achieved usingMestReNova software.

By way of example, a fixed cathode (e.g., stainless steel 2205) may beused in an electrochemical system where the electrolyte and/or catalystare altered to change the reaction compounds. In a modularelectrochemical system, the cathodes may be swapped out with differentmaterials to change the compounds.

Some embodiments of the present invention generally provide for newcathode materials, new electrolyte materials and new sulfur andoxygen-containing heterocyclic catalysts. Specific combinations ofcathode materials, electrolytes and catalysts may be used to get adesired organic compounds that may be used to efficiently purify(separate) carbon dioxide from other gases. Specific process conditionsmay be established that maximize the carbon dioxide conversion tooxalate. The oxalate may be evolved back into carbon dioxide at theanode and/or stored then evolved at a later time. A result may be apurer form of carbon dioxide gas than the original mixed gas.

Cell parameters may be selected to minimize unproductive side reactionslike H₂ evolution from water electrolysis. Choice of specificconfigurations of heterocyclic amine pyridine catalysts with engineeredfunctional groups may be utilized in the system 100 to achieve highfaradaic rates. Process conditions described above may facilitate longlife (e.g., improved stability), electrode and cell cycling and productrecovery. Heterocyclic amines related to pyridine may be used to improvereaction rates, product yields, cell voltages and/or other aspects ofthe reaction. Heterocyclic catalysts that contain sulfur or oxygen mayalso be utilized in the carbon dioxide reduction.

Some embodiments of the present invention may provide cathode andelectrolyte combinations for reducing carbon dioxide to organiccompounds in commercial quantities. Catalytic reduction of carbondioxide may be achieved using various cathodes. High faradaic yields(e.g., >20%) of organic compounds with steel and nickel alloy cathodesat ambient temperature and pressure may also be achieved. Copper-basedalloys used at the electrodes may remain stabile for long-term reductionof carbon dioxide.

Some embodiments of the present invention may provided for capturingcarbon dioxide from a mixture of gases. The capture may produce a purerform of carbon dioxide, oxalate, oxalic acids and/or oxalate salts. Thepurification generally consumes lower amounts of energy, have reactionrates high enough for scalability, and remains stable for long periods.

Various process conditions disclosed above, including electrolytechoice, cell voltage, and manner in which the mixed gas is bubbled,generally improve control of the reaction. Greater control over thereactions generally open the possibility for commercial systems that aremodular and adaptable to different situations.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the scope of the invention.

1. A method for purification of carbon dioxide from a mixture of gases,comprising the steps of: (A) bubbling said gases into a solution of anelectrolyte and a catalyst in an electrochemical cell, wherein (i) saidelectrochemical cell comprises an anode in a first cell compartment anda cathode in a second cell compartment, (ii) said cathode reducing saidcarbon dioxide into one or more compounds and (iii) said anode oxidizingat least one of said compounds into said carbon dioxide; and (B)separating said carbon dioxide from said solution.
 2. The methodaccording to claim 1, wherein said cathode material is at least one ofAl, Au, Ag, C, Cd, Co, Cr, Cu, Cu alloys, Ga, Hg, In, Mo, Nb, Ni, Nialloys, Ni—Fe alloys, Sn, Sn alloys, Ti, V, W, Zn, elgiloy, Nichrome,austenitic steel, duplex steel, ferritic steel, martensitic steel,stainless steel, degenerately doped p-Si, degenerately doped p-Si:As anddegenerately doped p-Si:B.
 3. The method according to claim 1, whereinsaid electrolyte is at least one of Na₂SO₄, KCl, NaNO₃, NaCl, NaF,NaClO₄, KClO₄, K₂SiO₃, CaCl₂, a H cation, a L1 cation, a Na cation, a Kcation, a Rb cation, a Cs cation, a Ca cation, an ammonium cation, analkylammonium cation, a F anion, a Cl anion, a Br anion, an I anion, anAt anion, an alkyl amine, borates, carbonates, nitrites, nitrates,phosphates, polyphosphates, perchlorates, silicates, sulfates, and atetraalkyl ammonium salt.
 4. The method according to claim 1, whereinsaid catalyst is one or more of amino-thiazole, aromatic heterocyclicamines with an aromatic 5-member heterocyclic ring, aromaticheterocyclic amines with 6-member heterocyclic ring, azoles,benzimidazole, bipyridines, furan, imidazoles, imidazole related specieswith at least one five-member ring, indoles, pyridines, pyridine relatedspecies with at least one six-member ring, pyrrole, thiophene andthiazoles.
 5. The method according to claim 1, wherein an energyconsumed by said purifying of said carbon dioxide is less than 500,000watt hours per ton of carbon dioxide.
 6. The method according to claim1, wherein said purification consumes approximately no additional waterafter said solution has been established in said electrochemical cell.7. The method according to claim 1, wherein said compounds comprise oneor more of oxalate, oxalate salts, organic acids, oxalic acid, glyoxylicacid and glyoxal.
 8. A method for purification of carbon dioxide from amixture of gases, comprising the steps of: (A) bubbling said gases intoa solution of an electrolyte in an electrochemical cell, wherein saidelectrochemical cell comprises an anode in a first cell compartment anda cathode in a second cell compartment; (B) reducing said carbon dioxideinto oxalate at said cathode; and (C) oxidizing said oxalate into saidcarbon dioxide at said anode.
 9. The method according to claim 8,further comprising the step of: separating said carbon dioxide from saidsolution.
 10. The method according to claim 8, wherein an energyconsumed by said purifying of said carbon dioxide is less than 200,000watt hours per ton of carbon dioxide.
 11. The method according to claim8, wherein said purification consumes approximately no additional waterafter said solution has been established in said electrochemical cell.12. The method according to claim 8, wherein an electrical potential ofsaid cathode is between zero volts and −1 volt.
 13. The method accordingto claim 8, wherein said reducing uses a single electron per carbonatom.
 14. The method according to claim 8, further comprising the stepof: pressurizing said electrochemical cell during said purification. 15.The method according to claim 8, wherein said electrochemical cellcomprises a divided electrochemical cell.
 16. A method for purificationof carbon dioxide from a mixture of gases, comprising the steps of: (A)bubbling said gases into a solution of an electrolyte in a dividedelectrochemical cell, wherein (i) said divided electrochemical cellcomprises an anode in a first cell compartment and a cathode in a secondcell compartment, (ii) said cathode reducing said carbon dioxide intoone or more of oxalate, organic acids and oxalate salts and (iii) saidanode oxidizing at least one of said one or more of oxalate, organicacids and oxalate salts into said carbon dioxide; and (B) separatingsaid carbon dioxide from said solution.
 17. The method according toclaim 16, wherein said oxidizing uses at most two electrons per carbonatom.
 18. The method according to claim 16, wherein said purificationconsumes approximately no additional water after said solution has beenestablished in said divided electrochemical cell.
 19. The methodaccording to claim 16, wherein an energy consumed by said purifying ofsaid carbon dioxide is less than 500,000 watt hours per ton of carbondioxide.
 20. The method according to claim 16, further comprising thestep of: adding to said solution one or more of (i) a hydrogeninhibitor, (ii) a heterocyclic compound and (iii) an alcohol.